Calcium/calmodulin-dependent protein kinase IV (CaMKIV) is a multifunctional serine/threonine kinase that is predominantly expressed in the brain and T-lymphocytes, where it appears to be important for synaptic plasticity and T cell activation, respectively. Published work from our laboratory has shown that the heterotrimeric serine/threonine protein phosphatase 2A (PP2A) forms a stable complex with CaMKIV, and negatively regulates its phosphorylation state both in vitro and in vivo. This complex is likely to be important for the control of processes such as neuronal long-term potentiation and T-cell activation, in which the phosphorylation of the cAMP response element binding protein (CREB) is controlled in a calcium-dependent manner by CaMKIV. The focus of this proposal is to elucidate the structure, function and regulation of the CaMKIV/PP2A signaling module. Special emphasis will be placed on defining the role of the PP2A regulatory B subunits in the assembly and regulation of this complex, since these subunits are thought to target PP2A holoenzymes to specific proteins in the cell. The in vitro experiments described in Specific Aim I will be used to determine the oligomeric B subunit composition of the CaMKIV/PP2A complex. The studies outlined in Specific Aim II and Specific Aim III examine the role of the PP2A regulatory B subunit in the assembly of the CaMKIV/PP2A complex, and in the regulation of CaMKIV signaling, respectively. Given the importance of CaMKIV in memory formation and immune responses, the proposed studies may provide insights and tools to further explore the role of the CaMKIV/PP2A signaling complex in the control of these biological processes.